1. Field of the Invention
The present invention relates to a sheet processing apparatus for performing a sheet processing such as a sort process and a staple process, and to an image forming system.
2. Description of the Related Art
Up to now, known is a sheet processing apparatus for performing a post-treatment such as a sort process and a staple process for a sheet delivered from an image forming apparatus or the like. In the sheet processing apparatus of this kind, the sheet delivered from the image forming apparatus or the like is received on an intermediate process tray to be subjected to the post-treatment such as the sort process and the staple process for a sheet stack on the intermediate process tray. The sheet stack subjected to the post-treatment is nipped by a pair of delivery rollers and is transported to a stack tray to be stacked (see U.S. Pat. No. 6,219,503).
In this case, the pair of delivery rollers is controlled to be in an open (i.e., spaced) state during the post-treatment for the sheet stack on the intermediate process tray, and to be in a closed state after the post-treatment is completed.
However, in conventional sheet processing apparatuses, a control of an opening amount of the pair of delivery rollers is not performed during the post-treatment for the sheet stack on the intermediate process tray, so the opening amount of the pair of delivery rollers is kept constant irrespective of a thickness of the sheet stack. As a result, even when the sheet stack is thin, there is required a certain period of time for an opening/closing operation of the pair of delivery rollers.
On the other hand, with an increase in processing speed of an image forming apparatus in recent years, the high processing speed is also demanded in the sheet processing apparatus. Thus, a waste of time such as a certain period of time required for the opening/closing operation of the pair of delivery rollers cannot be ignored any more, so it is demanded that the opening/closing operation of the pair of delivery rollers is performed swiftly to speed up the delivery process of the sheet stack.
To satisfy the demand, there is a possible way in which the opening/closing operation of the pair of delivery rollers itself is speeded up. However, even when the opening and closing operation of the pair of delivery rollers is simply speeded up, there arises a problem in that the pair of delivery rollers is bounced when the pair of delivery rollers is closed to nip the sheet stack, thereby causing vibration and noise.